[Fix] Catch symlink failure on Windows (open-mmlab#1039)

* Catch symlink failure on Windows

* Fix typo in setup.py

setup.py

@@ -1,5 +1,6 @@
 import os
 import os.path as osp
+import platform
 import shutil
 import sys
 import warnings
@@ -115,7 +116,10 @@ def add_mim_extension():
     # parse installment mode
     if 'develop' in sys.argv:
         # installed by `pip install -e .`
-        mode = 'symlink'
+        if platform.system() == 'Windows':
+            mode = 'copy'
+        else:
+            mode = 'symlink'
     elif 'sdist' in sys.argv or 'bdist_wheel' in sys.argv:
         # installed by `pip install .`
         # or create source distribution by `python setup.py sdist`